The use of fluids (and their compositions, characteristics, and functions) during the drilling of wells is well known. See for example the book "Composition and Properties of Oil Well Drilling Fluids," Fourth Edition, 1980, George R. Gray et al., Gulf Publishing Co., or Third Edition, 1963, Walter F. Rogers.
The use of fluids for conducting various operations in the boreholes of subterranean oil and gas wells which contact a producing formation are well known. Thus drill-in fluids are utilized when initially drilling into producing formations. Completion fluids are utilized when conducting various completion operations in the producing formations. Workover fluids are utilized when conducting workover operations of previously completed wells.
One of the most important functions of these fluids is to seal off the face of the wellbore so that the fluid is not lost to the formation. Ideally this is accomplished by depositing a filter cake of the solids in the fluid over the surface of the borehole without any loss of solids to the formation. In other words, the solids in the fluid bridge over the formation pores rather than permanently plugging the pores.
Many clay-free fluids have been proposed for contacting the producing zone of oil and gas wells. See for example the following U.S. patents: Jackson et al. U.S. Pat. No. 3,785,438; Alexander U.S. Pat. No. 3,872,018; Fischer et al. U.S. Pat. No. 3,882,029; Walker U.S. Pat. No. 3,956,141; Smithey U.S. Pat. No. 3,986,964; Jackson et al. U.S. Pat. No. 4,003,838; Mondshine U.S. Pat. No. 4,175,042; Mondshine U.S. Pat. No. 4,186,803; Mondshine U.S. Pat. No. 4,369,843; Mondshine U.S. Pat. No. 4,620,596; and Dobson, Jr. et al. U.S. Pat. No. 4,822,500.
These fluids generally contain polymeric viscosifiers such as certain polysaccharides or polysaccharide derivatives, polymeric fluid loss control additives such as lignosulfonates, polysaccharides or polysaccharide derivatives, and bridging solids.
It is well known that the polysaccharides are degraded by heat and agitation. Thus starch breaks down rapidly at temperatures of 200.degree. F. (93.degree. C.) and above, guar gum breaks down rapidly at temperatures above 150.degree. F. (65.5.degree. C.). The thermal degradation of carboxymethyl cellulose is accelerated as temperature approaches 300.degree. F. (150.degree. C.). As the polysaccharides degrade, the properties of the fluids containing them degrade. Thus fluids containing polysaccharide viscosifiers decrease in viscosity, and fluids containing polysaccharide fluid loss reducing additives exhibit increased fluid loss.
Jackson U.S. Pat. No. 3,852,201 discloses the use of magnesium oxide (magnesia) to stabilize fluids containing hydroxyethyl cellulose to temperatures as high as 275.degree. F. Jackson U.S. Pat. No. 3,953,335 discloses the use of magnesium oxide to increase the effectiveness of hydroxyalkyl cellulose over a higher temperature range and over a longer time period. Hartfield U.S. Pat. No. 3,988,246 discloses the use of magnesium oxide to stabilize fluids containing heteropolysaccharides such as xanthan gum. The wellbore fluids also included various starch derivatives as water loss reduction additives. Jackson U.S. Pat. No. 4,025,443 discloses the use of magnesium oxide to stabilize fluids containing hydroxyalkyl guar gum. Clarke-Sturman U.S. Pat. No. 4,900,457 discloses the use of a formate salt to enhance the thermal stability of aqueous polysaccharide solutions.